feat(linter): add support for nested config files

apps/oxlint/fixtures/nested_config/.oxlintrc.json

@@ -0,0 +1,6 @@
+{
+  "rules": {
+    "no-console": "error",
+    "no-debugger": "error"
+  }
+}

apps/oxlint/fixtures/nested_config/console.ts

@@ -0,0 +1 @@
+console.log("test");

apps/oxlint/fixtures/nested_config/debugger.js

@@ -0,0 +1 @@
+debugger;

apps/oxlint/fixtures/nested_config/package1-empty-config/.oxlintrc.json

@@ -0,0 +1,5 @@
+{
+  // this is a nested config file, but it should use the default config which
+  // is implicitly merged with this
+  "rules": {}
+}

apps/oxlint/fixtures/nested_config/package1-empty-config/console.ts

@@ -0,0 +1 @@
+console.log("test");

apps/oxlint/fixtures/nested_config/package1-empty-config/debugger.js

@@ -0,0 +1 @@
+debugger;

apps/oxlint/fixtures/nested_config/package2-no-config/console.ts

@@ -0,0 +1 @@
+console.log("test");

apps/oxlint/fixtures/nested_config/package2-no-config/debugger.js

@@ -0,0 +1 @@
+debugger;

apps/oxlint/fixtures/nested_config/package3-deep-config/.oxlintrc.json

@@ -0,0 +1,6 @@
+{
+  "rules": {
+    // should not be used, as it is overridden by the nested config
+    "no-console": "warn"
+  }
+}

apps/oxlint/fixtures/nested_config/package3-deep-config/src/.oxlintrc.json

@@ -0,0 +1,5 @@
+{
+  "rules": {
+    "no-console": "error"
+  }
+}

apps/oxlint/fixtures/nested_config/package3-deep-config/src/components/component.js

@@ -0,0 +1,3 @@
+export function Component() {
+  console.log("hello");
+}

apps/oxlint/src/lint.rs

@@ -9,10 +9,11 @@ use cow_utils::CowUtils;
 use ignore::{gitignore::Gitignore, overrides::OverrideBuilder};
 use oxc_diagnostics::{DiagnosticService, GraphicalReportHandler};
 use oxc_linter::{
-    loader::LINT_PARTIAL_LOADER_EXT, AllowWarnDeny, ConfigStoreBuilder, InvalidFilterKind,
-    LintFilter, LintOptions, LintService, LintServiceOptions, Linter, Oxlintrc,
+    loader::LINT_PARTIAL_LOADER_EXT, AllowWarnDeny, ConfigStore, ConfigStoreBuilder,
+    InvalidFilterKind, LintFilter, LintOptions, LintService, LintServiceOptions, Linter, Oxlintrc,
 };
 use oxc_span::VALID_EXTENSIONS;
+use rustc_hash::{FxHashMap, FxHashSet};
 use serde_json::Value;
 
 use crate::{
@@ -55,6 +56,7 @@ impl Runner for LintRunner {
             fix_options,
             enable_plugins,
             misc_options,
+            experimental_nested_config,
             ..
         } = self.options;
 
@@ -168,6 +170,56 @@ impl Runner for LintRunner {
 
         let number_of_files = paths.len();
 
+        // TODO(perf): benchmark whether or not it is worth it to store the configurations on a
+        // per-file or per-directory basis, to avoid calling `.parent()` on every path.
+        let mut nested_oxlintrc = FxHashMap::<&Path, Oxlintrc>::default();
+        let mut nested_configs = FxHashMap::<PathBuf, ConfigStore>::default();
+
+        if experimental_nested_config {
+            // get all of the unique directories among the paths to use for search for
+            // oxlint config files in those directories
+            // e.g. `/some/file.js` and `/some/other/file.js` would both result in `/some`
+            let mut directories = FxHashSet::default();
+            for path in &paths {
+                if let Some(directory) = path.parent() {
+                    // TODO(perf): benchmark whether or not it is worth it to produce the config files here without
+                    // iterating over the directories again. it might cause the cache for paths to be disrupted, but
+                    // it is unclear whether or not that is a problem in practice.
+                    directories.insert(directory);
+                }
+            }
+            for directory in directories {
+                // TODO: how should we handle a nested config error?
+                if let Ok(config) = Self::find_oxlint_config_in_directory(directory) {
+                    nested_oxlintrc.insert(directory, config);
+                }
+            }
+
+            // iterate over each config and build the ConfigStore
+            for (dir, oxlintrc) in nested_oxlintrc {
+                // TODO(perf): figure out if we can avoid cloning `filter`
+                let builder =
+                    ConfigStoreBuilder::from_oxlintrc(false, oxlintrc).with_filters(filter.clone());
+                match builder.build() {
+                    Ok(config) => nested_configs.insert(dir.to_path_buf(), config),
+                    Err(diagnostic) => {
+                        let handler = GraphicalReportHandler::new();
+                        let mut err = String::new();
+                        handler.render_report(&mut err, &diagnostic).unwrap();
+                        stdout
+                            .write_all(
+                                format!("Failed to parse configuration file.\n{err}\n").as_bytes(),
+                            )
+                            .or_else(Self::check_for_writer_error)
+                            .unwrap();
+                        stdout.flush().unwrap();
+
+                        return CliRunResult::InvalidOptionConfig;
+                    }
+                };
+            }
+        }
+
         enable_plugins.apply_overrides(&mut oxlintrc.plugins);
 
         let oxlintrc_for_print = if misc_options.print_config || basic_options.init {
@@ -245,8 +297,11 @@ impl Runner for LintRunner {
             }
         };
 
-        let linter =
-            Linter::new(LintOptions::default(), lint_config).with_fix(fix_options.fix_kind());
+        let linter = if experimental_nested_config {
+            Linter::new_with_nested_configs(LintOptions::default(), lint_config, nested_configs)
+        } else {
+            Linter::new(LintOptions::default(), lint_config).with_fix(fix_options.fix_kind())
+        };
 
         let tsconfig = basic_options.tsconfig;
         if let Some(path) = tsconfig.as_ref() {
@@ -381,6 +436,24 @@ impl LintRunner {
         Oxlintrc::from_file(&config_path).or_else(|_| Ok(Oxlintrc::default()))
     }
 
+    /// Looks in a directory for an oxlint config file, returns the oxlint config if it exists
+    /// and returns `Err` if none exists or the file is invalid. Does not apply the default
+    /// config file.
+    fn find_oxlint_config_in_directory(dir: &Path) -> Result<Oxlintrc, String> {
+        let possible_config_path = dir.join(Self::DEFAULT_OXLINTRC);
+        if possible_config_path.is_file() {
+            Oxlintrc::from_file(&possible_config_path).map_err(|e| {
+                let handler = GraphicalReportHandler::new();
+                let mut err = String::new();
+                handler.render_report(&mut err, &e).unwrap();
+                err
+            })
+        } else {
+            // TODO: Better error handling here.
+            Err("No oxlint config file found".to_string())
+        }
+    }
+
     fn check_for_writer_error(error: std::io::Error) -> Result<(), std::io::Error> {
         // Do not panic when the process is killed (e.g. piping into `less`).
         if matches!(error.kind(), ErrorKind::Interrupted | ErrorKind::BrokenPipe) {
@@ -862,4 +935,10 @@ mod test {
             vec![String::from("src/target"), String::from("!src/dist"), String::from("!!src/dist")]
         );
     }
+
+    #[test]
+    fn test_nested_config() {
+        let args = &["--experimental-nested-config"];
+        Tester::new().with_cwd("fixtures/nested_config".into()).test_and_snapshot(args);
+    }
 }

apps/oxlint/src/snapshots/fixtures__nested_config_--experimental-nested-config@oxlint.snap

@@ -0,0 +1,57 @@
+---
+source: apps/oxlint/src/tester.rs
+---
+########## 
+arguments: --experimental-nested-config
+working directory: fixtures/nested_config
+----------
+
+  x ]8;;https://oxc.rs/docs/guide/usage/linter/rules/eslint/no-console.html\eslint(no-console)]8;;\: eslint(no-console): Unexpected console statement.
+   ,-[console.ts:1:1]
+ 1 | console.log("test");
+   : ^^^^^^^^^^^
+   `----
+  help: Delete this console statement.
+
+  x ]8;;https://oxc.rs/docs/guide/usage/linter/rules/eslint/no-debugger.html\eslint(no-debugger)]8;;\: `debugger` statement is not allowed
+   ,-[debugger.js:1:1]
+ 1 | debugger;
+   : ^^^^^^^^^
+   `----
+  help: Delete this code.
+
+  ! ]8;;https://oxc.rs/docs/guide/usage/linter/rules/eslint/no-debugger.html\eslint(no-debugger)]8;;\: `debugger` statement is not allowed
+   ,-[package1-empty-config/debugger.js:1:1]
+ 1 | debugger;
+   : ^^^^^^^^^
+   `----
+  help: Delete this code.
+
+  x ]8;;https://oxc.rs/docs/guide/usage/linter/rules/eslint/no-console.html\eslint(no-console)]8;;\: eslint(no-console): Unexpected console statement.
+   ,-[package2-no-config/console.ts:1:1]
+ 1 | console.log("test");
+   : ^^^^^^^^^^^
+   `----
+  help: Delete this console statement.
+
+  x ]8;;https://oxc.rs/docs/guide/usage/linter/rules/eslint/no-debugger.html\eslint(no-debugger)]8;;\: `debugger` statement is not allowed
+   ,-[package2-no-config/debugger.js:1:1]
+ 1 | debugger;
+   : ^^^^^^^^^
+   `----
+  help: Delete this code.
+
+  x ]8;;https://oxc.rs/docs/guide/usage/linter/rules/eslint/no-console.html\eslint(no-console)]8;;\: eslint(no-console): Unexpected console statement.
+   ,-[package3-deep-config/src/components/component.js:2:3]
+ 1 | export function Component() {
+ 2 |   console.log("hello");
+   :   ^^^^^^^^^^^
+ 3 | }
+   `----
+  help: Delete this console statement.
+
+Found 1 warning and 5 errors.
+Finished in <variable>ms on 7 files with 98 rules using 1 threads.
+----------
+CLI result: LintFoundErrors
+----------

crates/oxc_linter/src/lib.rs

@@ -22,9 +22,14 @@ pub mod loader;
 pub mod rules;
 pub mod table;
 
-use std::{path::Path, rc::Rc, sync::Arc};
+use std::{
+    path::{Path, PathBuf},
+    rc::Rc,
+    sync::Arc,
+};
 
 use oxc_semantic::{AstNode, Semantic};
+use rustc_hash::FxHashMap;
 
 pub use crate::{
     config::{
@@ -62,11 +67,23 @@ pub struct Linter {
     options: LintOptions,
     // config: Arc<LintConfig>,
     config: ConfigStore,
+    // TODO(refactor): remove duplication with `config` field when nested config is
+    // standardized, as we do not need to pass both at that point
+    nested_configs: FxHashMap<PathBuf, ConfigStore>,
 }
 
 impl Linter {
     pub fn new(options: LintOptions, config: ConfigStore) -> Self {
-        Self { options, config }
+        Self { options, config, nested_configs: FxHashMap::default() }
+    }
+
+    // TODO(refactor); remove this when nested config is standardized
+    pub fn new_with_nested_configs(
+        options: LintOptions,
+        config: ConfigStore,
+        nested_configs: FxHashMap<PathBuf, ConfigStore>,
+    ) -> Self {
+        Self { options, config, nested_configs }
     }
 
     /// Set the kind of auto fixes to apply.
@@ -99,8 +116,15 @@ impl Linter {
         semantic: Rc<Semantic<'a>>,
         module_record: Arc<ModuleRecord>,
     ) -> Vec<Message<'a>> {
-        // Get config + rules for this file. Takes base rules and applies glob-based overrides.
-        let ResolvedLinterState { rules, config } = self.config.resolve(path);
+        // TODO(refactor): remove branch when nested config is standardized
+        let ResolvedLinterState { rules, config } = if self.nested_configs.is_empty() {
+            // Get config + rules for this file. Takes base rules and applies glob-based overrides.
+            self.config.resolve(path)
+        } else if let Some(nearest_config) = self.get_nearest_config(path) {
+            nearest_config.resolve(path)
+        } else {
+            self.config.resolve(path)
+        };
         let ctx_host =
             Rc::new(ContextHost::new(path, semantic, module_record, self.options, config));
 
@@ -181,6 +205,22 @@ impl Linter {
 
         ctx_host.take_diagnostics()
     }
+
+    /// Get the nearest config for the given path, in the following priority order:
+    /// 1. config file in the same directory as the path
+    /// 2. config file in the closest parent directory
+    fn get_nearest_config(&self, path: &Path) -> Option<&ConfigStore> {
+        // TODO(perf): should we cache the computed nearest config for every directory,
+        // so we don't have to recompute it for every file?
+        let mut current = path.parent();
+        while let Some(dir) = current {
+            if let Some(config_store) = self.nested_configs.get(dir) {
+                return Some(config_store);
+            }
+            current = dir.parent();
+        }
+        None
+    }
 }
 
 #[cfg(test)]